Materials which abrade readily in a controlled fashion are used in a number of applications, including as abradable seals. As will be appreciated by those skilled in the art, contact between a rotating part and a fixed abradable seal causes the abradable material to erode in a configuration which closely mates with and conforms to the moving part at the region of contact. In other words, the moving part wears away a portion of the abradable seal so that the seal takes on a geometry which precisely fits the moving part, i.e., a close clearance gap. This effectively forms a seal having an extremely close tolerance.
One particular application of abradable seals is their use in axial flow gas turbines. The rotating compressor or rotor of an axial flow gas turbine consists of a plurality of blades attached to a shaft which is mounted in a shroud. In operation, the shaft and blades rotate inside the shroud. The inner surface of the turbine shroud is most preferably coated with an abradable material. The initial placement of the shaft and blade assembly in the shroud is such that the blade tips are as close as possible to the abradable coating.
As will be appreciated by those skilled in the art, it is important to reduce back flow in axial flow gas turbines to maximize turbine efficiency. This is achieved by minimizing the clearance between the blade tips and the inner wall of the shroud. As the turbine blades rotate, however, they expand somewhat due to the heat which is generated. The tips of the rotating blades then contact the abradable material and carve precisely defined grooves in the coating without contacting the shroud itself. It will be understood that these grooves provide the exact clearance necessary to permit the blades to rotate at elevated temperatures and thus provide an essentially custom-fitted seal for the turbine.
In other gas turbines, the initial clearance is somewhat greater and the abradable coating is intended to protect the shroud and blade tips against wear during transient conditions (e.g., power surges).
In order for the turbine blades to cut grooves in the abradable coating, the material from which the coating is formed must abrade relatively easily without wearing down the blade tips. This requires a careful balance of materials in the coatings. In this environment, an abradable coating must also exhibit good resistance against particle erosion and other degradation at elevated temperatures. As known by those skilled in the art, however, these desirable characteristics have been difficult to obtain.
A number of abradable coatings have been proposed by others. These include cellular or porous metallic structures, such as illustrated in U.S. Pat. Nos. 3,689,971, 4,063,742, 4,526,509, 4,652,209, 4,664,973, and 4,671,735. Low melting point metallic coatings of indium, tin, cadmium, lead, zinc, and aluminum alloys have been suggested for use in providing "ablative" seals wherein heat generated by friction melts a clearance gap in the coating. This approached is exemplified in U.S. Pat. Nos. 2,742,224 and 3,836,156. Still others have proposed the use of hard ceramics such as ZrO.sub.2 and MgO for use in forming abradable coatings as shown in U.S. Pat. Nos. 4,405,284, 4,460,311, and 4,669,955.
In U.S. Pat. No. 3,508,955, a composite material is disclosed which comprises a porous metal impregnated with a fluoride of metals selected from Groups I and II of the Periodic Table of Elements. The use of fluoride salts and a barium fluoride-calcium fluoride eutectic is specifically mentioned as is the use of the material in bearings and seals. It is also disclosed therein that the resultant material can be sprayed with a surface layer of fluoride eutectic slurry which is then dried and sintered.
In U.S. Pat. No 4,867,639, abradable coatings for use in turbine or compressor shrouds are disclosed which are described as low melting fluoride compounds such as BaF.sub.2, CaF.sub.2 and MgF.sub.2 incorporated into a higher melting temperature ceramic or metallic matrix. It is disclosed that, alternatively, the soft ceramic phase may be used to fill or impregnate a honeycomb shroud lining made of the higher melting temperature hard ceramic or metal alloy, so that the soft ceramic is not eroded by hot gases in the turbine. Zirconia and/or alumina are disclosed as the preferred high melting temperature ceramic, and NiCr and NiCrAl are disclosed as preferred metals.
The use of metal matrix coatings having a plastic component such as a polyimide are also known for use in forming an abradable seal in high-efficiency compressors. Due to the lower temperatures generated in the compressor and the fact that the rotating blades are generally softer than those found in the turbine section, plastics have been used in lieu of solid lubricants such as CaF.sub.2. While the lower melting point of plastics is advantageous in such low temperature applications, the use of these coatings often results in the accumulation of residue on the rotating blades as well as a gradual increase in the gap between the blade and the coating because of thermal effects.
Therefore, it would be desirable to provide a composite material which abrades readily without producing significant wear of rotating parts.
It would also be desirable to provide such a material which can be fabricated using conventional thermal spray techniques.
It would still further be desirable to provide such a coating which could be used to form abradable seals in relatively low-temperature environments wherein the seal material does not adhere to rotating parts.
It would still further be desirable to provide a coating for forming abradable seals which can be custom formulated for a particular operating environment.
The present invention achieves these goals by providing thermal spray powders and composite coatings made with these powders which contain a matrix component, a solid lubricant component and a plastic component.